The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that binds diverse synthetic and natural compounds. We recently discovered that treating human orbital fibroblasts with AhR ligands not only impairs TGFb-driven myofibroblast formation, but also inhibits expression of the GPI-anchored protein, Thy1. This finding is important because human orbital fibroblasts are heterogeneous and can be divided into two subsets based on their surface expression of Thy1. Orbital fibroblasts expressing Thy1 (Thy1+) have the potential to form myofibroblasts, unlike cells lacking Thy1. And, the conversion of activated orbital fibroblasts to form scar-producing myofibroblasts is a key sight-threatening and incurable pathological feature of Thyroid Eye Disease (TED). Myofibroblasts produce excessive amounts of alpha-smooth muscle actin, collagen and hyaluronan leading to a stiff, fibrotic orbit. TED occurs in more than half of patients with Graves' Disease. Chronic orbital inflammation in TED leads to extensive tissue remodeling causing pain, proptosis, and even blindness. Current treatments for TED such as corticosteroids, external beam radiation and surgery are aimed at the consequences of disease, rather than the etiology, and cause unwanted side effects including post-surgical morbidity. Our long-term goal is to understand the mechanisms underlying TED leading to mechanism-based therapies. As there is no good animal model of TED, our translational studies use primary human orbital fibroblasts that accurately reproduce the events in the TED orbit. Identification of molecules that blunt TED myofibroblasts is a pressing need, and AhR ligands (e.g. ITE, FICZ) are a promising class of small molecules for potential use in TED. Goals: Establish the therapeutic value and the fundamental mechanisms whereby AhR ligands attenuate myofibroblast formation and extracellular matrix accumulation in TED. We will also discover the underlying mechanisms whereby AhR ligands repress Thy1 expression and determine if Thy1 repression is required for the anti-scarring properties of AhR ligands. Organizing Hypothesis: AhR ligands attenuate human orbital myofibroblast production and connective tissue remodeling in TED. Specific Aim 1: Test the prediction that key small molecule AhR ligands block human orbital fibroblast-to-myofibroblast differentiation. Specific Aim 2: Determine the mechanisms whereby AhR ligands block human orbital fibroblast-to-myofibroblast differentiation. Specific Aim 3: Investigate how AhR ligands mediate orbital myofibroblast formation by regulating Thy1 expression. Significance: Discovering that AhR ligands have therapeutic value in treating TED is a major advance, establishing new candidate drugs and targets for TED therapies. We will identify the mechanisms underlying Thy1 expression, myofibroblast formation and extracellular matrix accumulation in TED, and translate this knowledge into clinically relevant and innovative approaches to attenuate or even cure orbital remodeling and scarring in TED.